In the photo lithography technology, an exposure tool for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has hitherto been widely utilized. With the trend toward a higher degree of integration and a higher function of an integrated circuit, the refinement of the integrated circuit is advancing. The exposure tool is hence required to form a circuit pattern image with high resolution on a wafer surface at a long focal depth, and shortening of the wavelength of an exposure light source is being advanced. The exposure light source is further advancing from conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer layer (wavelength: 193 nm) is coming to be employed. Also, in order to cope with a next-generation integrated circuit whose circuit line width will become not more than 70 nm, an immersion lithography technique and a double exposure technique, each using an ArF excimer laser, are regarded as being leading. However, it is considered that even these techniques would be able to cover only the generation with a line width of up to 45 nm.
Under the foregoing technical trends, a lithography technique using, as an exposure light source, light having a wavelength of 13 nm to represent EUV light (extreme ultraviolet light) is considered to be applicable over generation of 32 nm and thereafter, and is attracting attention. The principle of image formation of the EUV lithography (hereinafter referred to as “EUVL”) is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The optical member for an exposure tool for EUVL includes a photomask and a mirror and is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate and (3) an absorber layer formed on the reflective multilayer. For the reflective multilayer, an Mo/Si reflective multilayer in which an Mo layer and an Si layer are alternately laminated is investigated; and for the absorber layer, as a film forming material, Ta and Cr are investigated. For the substrate, a material having a low thermal expansion coefficient is required so as not to generate a strain even under irradiation with EUV light, and a glass having a low thermal expansion coefficient or the like is investigated.
The TiO2—SiO2 glass is known as an extremely low thermal expansion material having a thermal expansion coefficient (CTE) lower than that of a silica glass. Also, since the thermal expansion coefficient can be controlled by the TiO2 content in glass, a zero-expansion glass whose thermal expansion coefficient is close to 0 can be obtained. Accordingly, the TiO2—SiO2 glass involves a possibility as a material to be used in an optical member for an exposure tool for EUVL.
According to the conventional preparation method of a TiO2—SiO2 glass, first of all, a silica precursor and a titania precursor are each converted into a gas phase and then mixed with each other. The mixture in a gas phase is introduced into a burner and thermally decomposed, thereby forming a TiO2—SiO2 glass particle. This TiO2—SiO2 glass particle is deposited in a refractory container and melted therein simultaneously with the deposition, thereby forming a TiO2—SiO2 glass.
Patent Document 1 discloses a method in which a TiO2—SiO2 porous glass body is formed and converted it into a glass body, and a mask substrate is then obtained.
The mirror to be installed in an exposure tool for EUVL must withstand the irradiation with EUV over a long period of time as 30,000 hours and involves a problem of a change in properties of the multilayer with a lapse of time. Main causes of the change with a lapse of time are contamination of carbon and oxidation of the multilayer due to an extremely small amount of water. A technique of using hydrogen is reported for the purpose of cleaning the contamination of carbon or reducing the oxidized multilayer (see, for example, Non-Patent Documents 1 and 2).
Patent Document 1: US-A-2002/157421
Non-Patent Document 1: S. Graham, C. Steinhaus, M. Clift, L. Klebanoff, and S. Bajt, Proceedings of SPIE, Vol. 5037, pp. 460-469 (2003)
Non-Patent Document 2: H. Oizumi, A. Izumi, K. Motai, I. Nishiyama, and A. Namiki, Japanese Journal of Applied Physics, Vol. 46, No. 22, pp.L633-L635 (2007)